This invention relates to apparatus for automatically interrupting current in an electrical circuit under overload conditions, and more particularly to current interrupters for use in relatively high current applications.
The current in electrical circuits must be interrupted automatically under overload or fault conditions, especially those caused by short circuits and the like, to prevent possible damage to the circuit components. In relatively high power equipment, such as power distribution and transmission apparatus used by utilities, the current must be limited in magnitude and interrupted very rapidly when an overload condition occurs, preferably within one quarter of a cycle, before the current reaches even one, amplitude peak.
Current limiting devices have been developed which are capable of limiting the current in about 200 microseconds, and interrupting it in less than one quarter of a cycle. Such devices generally include a fusible element which is placed in sand or the like. The fusible element includes one or more portions of reduced cross-section. An overload current melts the element at the portions of reduced cross-section, creating arcs. The sand absorbs enough energy from the arcs to extinguish them, and the current is interrupted. Such high voltage fuses have a relatively low continuous current carrying capability of about 200 amperes.
In one known type of current limiting interrupter with higher continuous current carrying capability, a large cross-section conductor or bus bar carries the current under normal operating conditions. When a fault such as a short circuit occurs, a linear pyrotechnic charge breaks the conductor into segments. In relatively low voltage applications, the sum of the arc voltage drops across the gaps thus created is sufficient to interrupt the current. In high voltage applications, the sum of these arc voltages would be too small to effect current limited interruption. Therefore, the current is commutated to a parallel current limiting fuse and the element of the current limiting fuse melts and causes current limited interruption in a conventional manner.
While current interrupters of the type described above have proven effective, they have been relatively expensive. A substantial portion of the cost of commercially available current interrupters of the above-described type is attributable to electronic equipment which is used to sense excessive current and ignite the explosive charge. This equipment may include isolation transformers, a current sensing transformer, and solid-state triggering logic. An external line voltage power source is generally needed for this equipment which further adds to the cost of the system and its installation. There is a need for a less expensive triggering system for sensing excessive current and detonating the pyrotechnic charge in pyrotechnic current interrupters. There is also a need for a triggering system which does not require external power sources or external connections. Past attempts to address these needs are disclosed in U.S. Pat. Nos. 4,538,133 and 4,479,105.
A general object of this invention is to provide new and improved apparatus for interrupting current in an electrical circuit.
A more specific object is to provide new and improved means for igniting a pyrotechnic charge in a current interrupter in response to excessive current flow.